Hydrogen retention properties of polycrystalline tungsten and helium irradiated tungsten

The hydrogen retention properties of a polycrystalline tungsten and tungsten irradiated by helium ions with an energy of 5 keV were examined by using an ECR ion irradiation apparatus and a technique of thermal desorption spectroscopy, TDS. The polycrystalline tungsten was irradiated at RT with energetic hydrogen ions, with a flux of 1015 H cm−2 s−1 and an energy of 1.7 keV up to a fluence of 5×1018 H cm−2. Subsequently, the amount of retained hydrogen was measured by TDS. The heating temperature was increased from RT to 1000°C, and the heating rate was 50°C min−1. Below 1000°C, two distinct hydrogen desorption peaks were observed at 200°C and 400°C. The retained amount of hydrogen was observed to be five times smaller than that of graphite, but the concentration in the implantation layer was comparable with that of graphite. Also, the polycrystalline tungsten was irradiated with 5 keV helium ions up to a fluence of 1.4×1018 He cm−2, and then re-irradiated with 1.7 keV hydrogen ions. The amount of retained hydrogen in this later experiment was close to the value in the case without prior helium ion irradiation. However, the amount of hydrogen which desorbed around the low temperature peak, 200°C, was largely enhanced. The desorption amount at 200°C saturated for the helium fluence of more than 5×1017 He cm−2. The present data shows that the trapping state of hydrogen is largely changed by the helium ion irradiation. Additionally, 5 keV helium ion irradiation was conducted on a sample pre-implanted with hydrogen ions to simulate a helium ion impact desorption of hydrogen retained in tungsten. The amount of the hydrogen was reduced as much as 50%.